This invention relates to electronic keyboard devices for which a keyboard defining overlay is utilized.
In recent years, a number of electronic devices have been created in which a plurality of pushbutton controls are arrayed on a common plane. Perhaps one of the most common uses of such devices is that in which a keyboard accompanies a computer interactive device or terminal. While the present invention is applicable to any system in which a group of depressible switches are arrayed in a flush mounting arrangement upon a common plane, it is of particular advantage in applications using the recently developed resilient keyboard systems.
Such resilient keyboard systems vary somewhat in specific construction but may be generally described as a sandwhich of three insulative resilient sheets. The outer two sheets have conductive patterns deposited on the inwardly facing surfaces while the third "middle" sheet is free of conductive patterns but instead defines a plurality of apertures. The apertures in the middle sheet are sized and positioned such that portions of the conductive patterns of the two outer sheets are spaced apart on opposite sides of the apertures. Externally applied pressure upon one of the outer sheets in the region of an aperture, for example when pressed by a human finger in a keyboard operation, deformes the sheets and causes the opposed conductive patterns to meet and make electrical contact. Because the structure is a very simple configuration and because the user exposed sheet of resilient material is a continuous sheet of material not mechanically interrupted by protruding switch buttons or keys, visual indicia of the key designations overlying the regions of the apertures show the locations of "keys".
In many applications of such keyboards however, it is desired that the system significance of some or all of the keys within the keyboard be differently defined under differing circumstances. For example, in one instance it may be desired to alphabetically code a keyboard. In another instance, numerical coding of the keyboard may be desired. In still another instance the same keyboard may be coded in a different language or even scientific symbols. This situation of multiple keyboard definition is encountered frequently in computer interactive systems in which a portion of the computer system is housed in a memory cartridge which is removable and therefore interchangeable with compatible cartridges. Often such systems include, in addition to interchangeable memory cartridges, accompanying resilient overlays having key definitions corresponding to the peculiar requirements of each cartridge.
When so used, it is desireable that, once inserted, the resilient overlay be retained in a relatively secure manner to avoid displacement or damage during keyboard operation. However, it is also desireable that the means of retention of the flexible overlay in such systems be conducive to easy removal and insertion. While a number of overlay retention schemes have been devised and have to various degrees been successful in meeting these criteria, there remains in the art the need for an interchangeable keyboard defining system which provides secure overlay positioning and retention as well as easy removal and insertion.